In the past, an electrostatic capacity coupling system has been proposed for a digitizing apparatus for a tablet using a cordless pen. According to this system, an active circuit including a battery is encased in a pen, an unbalanced AC signal is transmitted, and a coordinate is detected from the intensity of a single ended AC signal received by grid conductors within the tablet through a capacity coupling. One example of such a system as described is disclosed in U.S. Pat. No. 4,672,154 issued to James L. Rodgers on Jun. 9, 1987, which makes use of a virtual ground shield. In such an electrostatic capacity coupling system as described, since the unbalanced signal is transmitted, foreign noises are often mixed, the S/N ratio of signals is poor, and a relatively intense AC signal is transmitted from the pen, thus increasing the number of batteries used in the pen and accordingly increasing power consumption. Further, the pen is heavy and the construction of the tablet conductors is also complicated.
There has been known a further apparatus using an electromagnetic coupling system in which only a resonance circuit which is a passive circuit is contained in a pen. In this system, when a metal, such as a metal ring worn by an operator, is placed in contact with the tablet, erroneous operations tend to occur. Further, when this system is combined with a TFT color liquid crystal display for use, erroneous operations also occur due to interference.